1. Field of the Disclosure
The present invention relates to liquid crystal display devices, and more particularly, to a prism sheet which is made to improve lamp mura, and a back light unit and a liquid crystal display device therewith.
2. Discussion of the Related Art
As an information oriented society is developed, demands for display devices have been increased in a variety of forms. To meet the demands, various flat display devices including liquid crystal display devices LCD, plasma display devices PDP, electro-luminescence display devices ELD, have been studied and some of which are used widely as display device, already.
Of the flat display devices, currently, the liquid crystal display device replaces the cathode ray tube CRT rapidly owing to advantages of a good picture quality, light weight, thin, low power consumption, and so on. The liquid crystal display device is developing as a monitor of a notebook computer, display panel of a TV set, and so on.
The liquid crystal display device has a back light unit mounted to a back side of a liquid crystal display panel, for displaying a desired image by cutting/passing a light from the back light unit by controlling a switch for each of pixels on the liquid crystal panel.
In the back light unit, there are an edge lighting type back light unit which has a light source at an edge of the liquid crystal display panel, and a direct lighting type back light unit which has a plurality of light sources arranged at regular intervals.
Presently, the direct lighting type back light unit is developing to be slim by reducing an overall thickness of the back light unit while providing a minimum gap between the lamps and sheets.
Since such back light unit has many advantages in view of system, producers of the liquid crystal display device put many efforts for developing a slim back light unit.
FIG. 1 illustrates a related art back light unit, schematically.
Referring to FIG. 1, the related art liquid crystal display device is provided with a cover bottom 10, a reflective plate 20 formed on an inside surface of the cover bottom 10, a plurality of lamps 30 arranged on the reflective plate 20 at regular intervals for emitting a light, a diffusion plate 40 over the lamps 30 for diffusing the light, a plurality of optical sheets 50 on the diffusion plate 40 for scattering the light diffused thus, a mold frame 60 which surrounds the cover bottom 10 and edges of the optical sheets 50, a liquid crystal display panel 70 over the diffusion plate 40 seated on an edge of the mold frame for displaying the image, and a case top 80 for fastening the mold frame 60 and the liquid crystal display panel 70.
The optical sheets 50 has two diffusion sheets, a protective sheet, and two prism sheets, and may have a polarized light separation sheet depending on products.
In the meantime, currently, in fabrication of the liquid crystal display device having the directing lighting type back light unit applied thereto, producers put much efforts in fabricating a larger, slimmer, and lower cost liquid crystal display device, by removing the sheets.
With regard to the distances between the reflective plate 20 and the lamps 30, and between the lamps 30 and the diffusion plate 40, lamp mura is shown if the distances are made shorter or a number of the lamps are reduced for reducing an overall thickness, and the lamp mura is shown even in a case the optical sheets are removed, selectively.
FIG. 2 illustrates a diagram of the prism sheet in a related art back light unit.
Referring to FIG. 2, the related art prism sheet 60 is provided with a diffusion film 62, and a plurality of prism mountains 63 on the diffusion film 62.
Each of the prism mountains 63 has a shape of a triangular column arranged such that an apex thereof faces the diffusion sheet 61. The prism mountains 63 scatters the light from the lamps toward the diffusion sheet 61.
In the meantime, the prism sheet 60 converges a light diverging in an X-axis direction better or diverging in a Y-axis direction better depending on a direction of arrangement of the prism mountains 63.
In detail, the prism sheet 60 scatters the light diverging in a direction sloped surfaces of the prism mountains 63 face, i.e., in the Y-axis direction in the drawing, efficiently. Opposite to this, the prism sheet 60 scatters the light diverging in a length direction of the prism mountains 63, i.e., in the Y-axis direction in the drawing, poorly.
According to this, the light passed through the prism sheet 60 spreads in the Y-axis direction less (due to good converging), and in the X-axis direction more (due to poor converging).
Thereafter, the light passed through the prism sheet 60 thus is incident on a back side of the liquid crystal display panel via the diffusion sheet 61. In this instance, due to a difference of converging efficiency depending on the directions, a picture quality can be poor due to the light incident on a back side of the liquid crystal display panel.
FIG. 3 illustrates a diagram for explaining problems taking place in the related art back light unit.
Referring to FIG. 3, with regard to the distances between the reflective plate 20 and the lamps 30, and between the lamps 30 and the diffusion plate 40 shown in FIG. 1, lamp mura is shown if the distances are made shorter or a number of the lamps are reduced for reducing an overall thickness, and the lamp mura is also shown even in a case the optical sheets are removed, selectively.
The lamp mura is shown in a case emission angles of the lights from the lamps overlap with each other, making an overlapped portion brighter than surroundings. That is, in order to make the emission angles not to overlap, a certain distance is required between the lamps and the diffusion plate, and if the distance is shorter, problems with respect to an outer appearance, such as the lamp mura, takes place.